This invention relates to optical analyzers and more particularly to an optical analyzer and a method of using the analyzer to analyze particulate matter, especially carbonaceous material.
The present invention is an extension of the optico-thermal method of analysis of light absorbing graphitic or black carbon in aerosol particulate material. Such material collected from ambient air or combustion sources always contains carbonaceous particles which are black or grey when collected on filters. In the optico-thermal method of analysis, a sample of particulate matter is collected on a clear quartz fiber filter. The sample is then placed in a combustion tube and heated at a controlled rate from room temperature to a temperature above the combustion temperature of the carbon in the material. A constant gas flow of an oxidizing atmosphere, ambient air or oxygen, is maintained by the sample during the heating.
As the sample is heated, decomposition of the carbon compounds in the sample will occur and carbon dioxide will be given off. The carbon dioxide concentration of the atmosphere leaving the sample is continuously monitored and the CO.sub.2 concentration is plotted out on a graph versus the temperature of the sample. The plotted trace is commonly referred to as a "thermogram." With a constant gas flow and a linear rise in temperature of the sample, the area under the thermogram is proportional to the total mass of carbon in the sample.
Oftentimes, however, the thermogram will have a number of CO.sub.2 peaks at different temperatures during the heating of the sample. For example, in addition to the CO.sub.2 peak corresponding to the combustion of black carbon, there may be CO.sub.2 peaks corresponding to volatilization and incomplete combustion of material which is not optically absorbing, or corresponding to combustion or pyrolysis of material which also is not optically absorbing. When the thermogram has these additional CO.sub.2 peaks, the area under the thermogram is no longer proportional to the total mass of just the black carbon. In order to determine which CO.sub.2 peak of the thermogram corresponds to the combustion of the black carbon in the sample, the light passing through the sample is detected and the degree of optical transmission of the sample is plotted out on the same chart with the thermogram. At some point during the heating, typically in the vicinity of 470 degrees C., the black carbon will combust and burn off with an accompanying sharp increase in the light transmission of the sample. This easily identified increase in optical transmission is used to identify the peak of the thermogram occurring during combustion of the black carbon. The area of the thermogram under this particular CO.sub.2 peak thus provides the desired information as to the amount of black carbon in the sample.
The above-described optico-thermal method is effective in the identification of black carbon, but is of limited utility for other analyses. Further, the monitoring of the CO.sub.2 concentration requires a relatively long period of time for a run. For example, the rate of heating is typically in the order of 10 C. degrees/min. Thus, a run, starting at room temperature and ending at about 700 C., will take over an hour.